The field of the method according to the invention is the study of the properties of the particles in a medium, such as a gas or a liquid. The field of the device according to the invention is the study of the properties of the particles contained as particles of solids in gases, such as flue gases. The device according to the invention is also suitable for studying the properties of particles of relatively clean gases, such as clean room and occupational air.
The properties of particles in gases and liquids are generally measured by observing the scattering of a ray of light as it travels through a medium. When studying flue gases, the measuring device is generally located in a flue, so the particles are passing the measurement area which is prone to misalignment and thus may lead to inaccurate measurements. A laser is generally used as the source of light when forming the ray of light.
Problems are caused by the contamination of the measuring device by impurities and moisture in the flue gas. Contamination of the optical elements causes variations in the intensity of light. Due to contamination, compensation is required to achieve the correct measurement results. Attempts have been made to prevent contamination of the optical elements by guiding the ray of light to the flue gas and away from it through an opening, from which a clean gas, such as air, is blown to flow away from the optical elements and thus prevent the flue gas from meeting the element. This arrangement endangers the accuracy of the measurement, because the blown clean gas dilutes the flue gas and causes unintended refractions caused by purge air, flue gas layers and temperature gradients.
A laser produces monochromatic light, the scattering angle of which depends on the wavelength of the light. The wavelength of the light produced by a laser depends on the temperature. In addition, the intensity of the light produced by a laser depends on the light source and electronics temperature. Temperature compensation is therefore needed. The scattering angle and intensity of the monochromatic light produced by a laser changes as the size of the particles changes. Both the wavelength of the scattered light and the intensity of the light depend on the size and colour, and therefore the reflectivity of the particles.
It is known to study the particles contained in a medium by using as a light source a light source emitting several wavelengths, for example a LED (Light Emitting Diode) light source emitting white light. As the light emitted by the light source contains several wavelengths, the wavelength of the light scattered by the particles will not significantly depend on the size of the particles.
From the aforementioned physical phenomena it follows that various data can be measured, decided on, and calculated based on the intensity, scattering angle, and wavelength of the light scattered from the particles in a medium. For example, it is possible to determine the concentration of the particles, the particles' size, and the number of particles.
A conventional particle-measuring device, which is located in a flue, and uses a laser as a light source, is large and heavy. It requires access for maintenance and cleaning. The art is represented by patent publication U.S. Pat. No. 4,024,407.
EP 1969997 and EP 0768521 disclose light absorption-based measurement methods and devices for the measurement of gases. The documents also disclose ways to extend the path of light to enhance absorption and preventing scattered light from entering the absorption sensor.
It is known to place the measuring device in a small-dimension flue, a measuring duct, parallel to the actual flue. A conventional measuring device located in a measuring duct requires a measuring chamber around it that is significantly wider than the typical pipe size of a measuring duct. A flue-gas flow, corresponding to that in the actual flue, is attempted to be arranged in the measuring chamber.
Accurate and reliable measurement requires not only the measurement of scattered light but also the measurement of unscattered light. The measurement of unscattered light is required, for example, for calibrating the measuring device and monitoring the contamination of the measuring device's optics.